If the fuel tank wasn't completely full, and had air at the top, when it gets hot outside this air expands and creates pressure. When it exceeds the "blow off" pressure of the valve built into the fuel cap, it vents the pressure to the outside (as it is designed to do), and this smells (very strongly) of gas, because it is.

We get people posting about this every time it starts to get hot. Park your bike with a full gas tank, and you shouldn't have this issue.